If you want to have a lot of kids, try to refrain from having them closer than 2-3 years apart. It takes awhile for the mother's body to renourish and strengthen up. Vitamin A and a strong thyroid/iodine are the keys to fertility.
Miscarriages are often not due to physical causes. For whatever reason, the incoming child - or even the parent- decides that this is not the right situation for their evolution, so they separate and the earthly form is no longer needed. I wish more religions were open to the idea of reincarnation and soul choices, it would spare the mothers a lot of grief and guilt over perceived deficiencies that simply are not there.